CAYCE, S.C. (AP) — Federal investigators spent a second day Monday at the site of a deadly train crash in South Carolina, where an Amtrak train was mistakenly sent off a main track and down a side spur – and into a parked freight train. The ensuing crash early Sunday killed two people and injured more than 100 passengers.

The investigation headed by the National Transportation Safety Board could take years, but federal investigators are already focusing on one critical factor: that a switch, set manually, may have caused the passenger train to barrel down the wrong track.

Moreover, authorities say the crash could have been prevented with a GPS-based system called “positive train control,” which knows the location of all trains and the positions of all switches in an area and is designed to prevent two trains from traveling on the same track at the same time.

“It could have avoided this accident. That’s what it’s designed to do,” said National Transportation Safety Board Chairman Robert Sumwalt.

Regulators have demanded the implementation of positive train control for decades, and the technology is now in place in the Northeast, but railroads that operate tracks used by Amtrak elsewhere in the U.S. have won repeated extensions from the government. The deadline for installing such equipment is now the end of 2018.

CSX Corp. — the freight railroad operator which runs that stretch of track — issued a statement expressing condolences but said nothing about the cause.

“Business as usual must end,” Democratic Sen. Richard Blumenthal of Connecticut said after this latest crash.

Sumwalt said the passenger train hurtled down a side track near Cayce (CAYsee) around 2:45 a.m. Sunday after a stop 10 miles (16 kilometers) north in Columbia because a switch had been locked in place, diverting it from the main line. A crew on the freight train had moved the switch to drive it from one side track — where it unloaded 34 train cars of automobiles — to the side track where it was parked. The switch was padlocked as it was supposed to be, Sumwalt said.

The system that operates the train signals in the area was down, so CSX dispatchers were operating them manually. Sumwalt said it was too early to know if the signal was red to warn the Amtrak crew that the switch was not set to continue along the main train line.

Just hours after Sunday’s crash, which also sent 116 of the 147 people on board the New York-to-Miami train to the hospital, Amtrak President Robert Anderson said there must be no more delays from the federal government in installing the safety system by the end of 2018.

He deferred to investigators about whether the system would have stopped this crash. “Theoretically, an operative PTC system would include switches in addition to signals, so it would cover both speed and switches,” Anderson said.

The Silver Star was going an estimated 59 mph (94 kph) when it struck the freight train, Gov. Henry McMaster said. It was the middle of the night, and many people were jolted from sleep by the crash and forced into the cold.

“I thought that I was dead,” said passenger Eric Larkin, of Pamlico County, North Carolina, who was dazed and limping after banging his knee.

Suddenly awake, Larkin said the train was shaking and jumping and his seat broke loose, slamming him into the row in front of him.

He heard screams and crying all around him as he tried to get out. Other passengers were bleeding.

The locomotives of both trains were crumpled on impact, and the Amtrak engine ended up on its side. One car in the middle of the Amtrak train was snapped in half, forming a V off to one side of the tracks.

Engineer Michael Kempf, 54, of Savannah, Georgia, and conductor Michael Cella, 36, of Orange Park, Florida, were killed, Lexington County Coroner Margaret Fisher said. Of the 116 people taken to four hospitals, only eight remained in the hospital Monday. Two people were in critical condition, but most other injuries were limited to cuts, bruises and whiplash, authorities said.

“Any time you have anything that happens like that, you expect more fatalities. But God blessed us, and we only had the two,” Fisher said, her voice choked with emotion.

On Wednesday, a chartered Amtrak train carrying Republican members of Congress to a retreat slammed into a garbage truck at a crossing where locals said the safety arms were down even when no trains approached. That accident in rural Virginia killed one person in the truck and injured six others.

And on Dec. 18, an Amtrak train ran off the rails along a curve during its inaugural run near Tacoma, Washington, killing three people and injuring dozens. It was going nearly 80 mph (128 kph), more than twice the speed limit.

“It’s becoming almost like an epidemic for Amtrak,” said Najmedin Meshkati, a University of Southern California engineering professor who has studied positive train control.

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Associated Press writers Jeffrey Collins in Columbia, South Carolina, and Michael R. Blood in Los Angeles contributed to this report along with AP video journalist Josh Replogle.